NO812489L - PROCEDURE FOR EXTRADUCING LINEAR ETHYLENE COPOLYMERS WITH SNAR MOLECULE WEIGHT DISTRIBUTION - Google Patents

Description

The present invention relates to a method for extruding molten linear ethylene copolymers with a narrow molecular weight distribution.

Conventional low-density polyethylene has until now been polymerized in thick-walled autoclaves or tube reactors at pressures up to 3500 kg/cm<2> and temperatures up to 300°C. The molecular structure of high pressure low density polyethylene (HP-LDPE)

is very complex. Permutations in the arrangement of the simple building blocks are virtually endless. HP-LDPE is characterized by an intricate, long-chain, branched molecular architecture. These long chain branches have a strong effect on the melt rheology of these resins. HP-LDPE also has a spectrum of short-chain branches, typically 1 to 6 carbon atoms in length. These short-chain branches interrupt crystal formation and reduce resin density.

In recent developments in low-pressure technology, low-density linear ethylene polymers with a narrow molecular weight distribution can now be produced at low pressures and temperatures by copolymerizing ethylene with various alpha blefins. These low pressure LDPE (LP-LDPE) resins typically have little if any long chain branching. They are short-chain branched with branch lengths and frequencies that are regulated by the type and amount of comonomer used during the polymerization.

US-Serial No. 892,325, filed again under Serial No. 014,414 corresponding to European patent application no. 79100953.3 published as publication no. 004,645 on 17 October 1979 describes that ethylene copolymers with a density of 0.91 to 0.96, a melt flow ratio of -22 to -32 and with relative catalyst residual content can be produced in granular form at relatively high productivities if the monomer(s) are copolymerized in a gas phase process with a Mg-Ti containing complex catalyst with specific high activity, mixed with an inert support material.

US-Serial No. 89 2.322, submitted again as Serial No. 012,720 corresponding to European Patent Application No. 79100958.2 published under No. 004,647 on October 17, 1979 describes that ethylene copolymers with a density of from 0.91 to 0.96, a melt-> <

flow ratio from -22 to -32 and a relatively low catalyst residue content can be produced in granular form. at relatively high

productivities if the monomer(s) are copolymerized in a gas phase process with a Mg-Ti containing complex catalyst with specific high activity, impregnated in a porous inert support material.

US-Serial No. 89 2.0 37, refiled under No. 014,412 corresponding to European Patent Application No. 79100957.4, published under No. 004,646 on October 17, 1979 describes that ethylene copolymers with a density of from -0.958 to ^-0.972

and a melt flow ratio from -22 to "^32 and with relatively low catalyst residual content can be produced at relatively high productivities for commercial purposes by a low-pressure gas phase process if ethylene is homopolymerized in the presence of a Mg-Ti containing complex catalyst with high activity, mixed with an inert carrier material. The granular polymers thus produced are useful for a number of end uses.

The polymers produced e.g. in the method in the aforementioned applications using Mg-Ti containing complex catalysts has a narrow molecular weight distribution Mw/Mn of approx. -2.7

to -4.1.

Over the years, film extrusion equipment has been optimized with regard to the rheology of HP-LDPE. The different molecular architecture of low pressure low density polyethylene (LP-LDPE) results in a film processing behavior that requires different extrusion parameters. A common conventional extruder screw commonly used for HP-LDPE comprises an elongated cylinder that can be heated or cooled at various points along its length and a screw that extends longitudinally through the cylinder. The screw has a threading on the surface which cooperates with the cylindrical inner surface of the cylinder and provides an elongated screw worm channel. Although the pitch of the thread can vary along the length, it is currently common to use screws with a constant pitch where the pitch is "quadratic", i.e. that the distance between adjacent threaded wings is equal to the diameter. The screw is turned around its own axis to process plastic material and feed this towards the discharge end of the cylinder.

An extruder screw usually has a number of sections that have a configuration particularly suited to fulfill particular functions. Examples are "feeding" section and "dosing" section, these are of principle importance and are present in almost all extruders for processing the thermoplastic polymers.

A typical feed section of an extruder screw stretches

below and forward from a feed opening where polymer in pellet or powder form is fed to the extruder to be fed along the inside of the cylinder by means of the screw feed section. In this section, the channel depth of the screw is usually large enough

to exceed the solid polymer. This is the desired effect because the overfeed action serves to compress the polymer particles and pressurize them, thus forming a solid bed of advancing material.

The processing of the material that creates heat and the melting of the polymer occurs as the material moves along the feed section of the screw. Most of the melt enters near the cylinder surface on the inside of this between a thin molten film and the solid polymer layer. This general pattern is maintained until a significant proportion of the polymer reaches the molten state. After approx. 40-70% of the polymer is melted, a breakdown of the solid layer usually occurs and during this time the particles of solid polymer are dispersed in the polymer melt.

From this point it is often advantageous to thoroughly mix the polymer melt with unmelted material to accelerate melting and to minimize local inhomogeneities.

The "dosing" section of an extruder screw has the special function of exerting a pumping action on the molten polymer. Generally, the throughput achieved by a screw is assumed to be a function of the combination of "draft flow" and "pressure flow" in the metering section.

The draft current is usually the current that results

from the relative movement between the screw and the inner surface of the extruder barrel. It can be considered to be proportional to the product of the mean relative velocity and the channel cross-sectional area. This draft current component is directed towards the outlet end of the screw. It can be increased by increasing the screw speed and/or by increasing the depth of the flow channel in the screw.

Oppositely directed to the draft flow, there is a pressure-flow component which originates from the material's resistance to flow through the narrowed outlet at the end of the extruder passage. The speed of the screw does not directly affect the pressure flow component, but of course it can be affected by factors such as return pressure and material viscosity, factors which in turn significantly affect the pressure flow component. On the other hand, the pressure flow is directly affected by both the depth and the length of the screw channel; an increase of the channel depth tends to strongly increase the pressure flow component and an increase of the channel length tends to reduce this return flow component.

In addition to the principle "feeding" and "dosing" sections, an extruder screw may also comprise a number of other separate sections. Almost all screws include e.g. so-called "transfer" sections.

Over the years there has been a trend towards the use of extruders capable of high output quantities. For many applications, various economic considerations in manufacturing are possible where high extruder outputs can be achieved on a reliable basis.

Although LP-LDPE resins can be extruded on equipment designed for HP-LDPE resins as described above

certain equipment modifications are often necessary to extrude low pressure resins under optimal conditions and at speeds comparable to those of high pressure resins. Detergent is particularly important during extrusion of LP-LDPE, which is then further reprocessed into film. The problem seems to be that when new granular low pressure resins are extruded through equipment designed for earlier high pressure resin pellets they tend to give lower pass rates. When they are pushed to higher speeds, operation becomes unstable. An analysis of the problem showed that the above stated deficiencies stemmed from "starvation", i.e. incomplete filling of the dosing section of the extruder screw due to different solids and melt rheological properties of LP-LDPE.

Accordingly, the current methods for extruding LP-LDPE resins are not entirely satisfactory from a practical commercial point of view and there is today a need to provide an extrusion method which provides additional compression {necessary with granular materials due to of lower mass densities) without significantly deepening the feed section and weakening the screw used for extrusion.

A general object of the invention is to provide an improved method for dealing with the above stated problems.

Another object of the invention is to provide a method for achieving good extrudate dosing performance at high speeds but without irregularities in the consistency of the material being extruded.

These and other objects are achieved by a process for extruding granular low-density linear ethylene polymers with a narrow molecular weight distribution which comprises feeding the granular polymers into an extrusion apparatus comprising an extruder screw, the extruder screw having a feed section into which the polymers are fed and then passed through a transfer section and a dosing section where the thread distances in the feed section are essentially the same and the thread distance in the dosing section is essentially the same and where the thread distance in the feed section is 5 to 50% greater than the thread distance in the dosing section.

According to the invention, the thread distance change between the feed section and the dosing sections is carried out by means of one abrupt (during one revolution or less) change in the transition section or alternatively at the beginning of the transition section or at the beginning of the dosing section.

A better understanding of this and other features and advantages of the invention will be apparent from consideration of the following description of certain embodiments shown in the accompanying drawings. Fig. 1 shows an extruder screw of an embodiment according to the invention where the thread distance change occurs at the beginning of the transition section; Fig. 2 is essentially fig. 1, but shows a second embodiment of the invention where the thread distance change between the feeding section and dosing section is carried out by a

abrupt change at the beginning of the dosing episode; and

fig. 3 is substantially similar to fig. 1, but shows a third embodiment of the invention in which the abrupt change between the feeding section and dosing section is carried out by an abrupt change in the transition section.

Although Figures 1 to 3 only show an extruder screw for the purpose of carrying out the method according to the invention, it will be clear that the extruder screw is used in its usual environment, i.e. in cooperative connection with conventional frame devices, a horizontal extruder cylinder, feeding and bin devices and drives which are not shown because, as previously mentioned, they constitute well-known apparatus features which are commercially available. As illustrative examples, the extruder screw can be arranged in an apparatus such as described in US-PS 4,155,655 and US-PS 4,053,143.

The screw 10 as shown in fig. 1 is a single-stage screw

with threading 11 and comprises a feeding section 12, a transition section 14 and a dosing section 16. The feeding section 12 receives granular polymer for softening and extrusion in the apparatus. In this section, the radius of the screw root 18 is smaller than the radius

in the transition section and the dosing section as shown in the drawings. Thus, the depth of the material channel between the screw root and the inner surface of the extruder cylinder is large. The large channel depth gives the feed section of the screw a high material delivery rate and causes the particles or pellets of solid polymer to be pressed together. Turning the screw helps the solid layer of plastic generate heat which serves to melt the solid material.

The feed section 12 is shown in fig. 1 with a length approximately five times the nominal screw diameter. In this section, the depth of the material channel is essentially constant.

Downstream of the feed section 12 there is a so-called "transition" section shown in fig. 1 with a length of approx. 4 to 5 times the nominal screw diameter. In this zone, the screw radius is gradually increased so that the helical polymer channel becomes progressively narrower. The melting of the material continues as it moves in the transition section 14. Melting mostly occurs at an interface between a film of molten polymer and a solid layer of packed polymer particles. As the melting continues, however, a point is reached where the solid layer is broken up and small particles of solid polymer are dispersed in the mass of molten polymer.

The final processing and pumping of the polymer is achieved in the dosing section 16 of the screw 10 shown in fig. 1.

This is the screw section that delivers high quality polymer melt to a nozzle in a uniform amount. It should be clear that in this section the screw radii remains constant and is greater than the screw radii in the feed section. Furthermore, it will appear

that although the dosage section has a length of approx. 8 times the diameter, it can be smaller or larger, all depending on the total length of the screw.

Under conventional methods for the production of HP-LDPE, an extruder screw is usually used in which the thread distance in each section has, in principle, the same dimension. When processing LP-LDPE granular resins according to the invention, however, problems arise in the dosing section when using conventional methods due to the material's lower mass density. It was found that these problems could be overcome by increasing the thread spacing in the feed section, which would provide the additional compression necessary to process the particular materials. With reference to fig. 1, it will thus appear that the thread distance which is usually indicated by the reference number 20 in the feed section 12 and which runs from a point on a thread wing of the screw to a corresponding point on the adjacent thread is essentially of the same dimension through the feed section. In the dosing section 16, where the distance is usually indicated with the reference number 12, it is also essentially the same dimension.

In the feeding section 12, however, the distance is 5 to 50% greater and preferably 15 to 30% greater than the distance in the dosing section 16.

As stated above, the distance change between the feeding section 12 and the dosing section 16 can be carried out by an abrupt

(during one revolution or less) change in the transition section or alternatively at the beginning of the transition section-

tet or at the beginning of the dosing section.

Fig. 1 shows that the abrupt change occurs at the beginning of the transition section at the distance which is generally indicated by the reference number 24. With reference to figures 2 and 3 where like parts are given the same reference numbers, it appears that in fig. 2, the change which is generally indicated by the reference number 26 occurs at the beginning of the dosing section 16 and thus there will be somewhat smaller threads in the transition section. In fig. 3, the change generally indicated by the reference number 28 occurs in the middle of the transition section.

Certain exemplary dimensions will serve to further illustrate the nature of the single-stage screw which is used according to the invention and which is illustrated in the drawings.

The extrusion screws used according to the invention can have a length:diameter ratio of 15:1 to 30:1 and preferably 17:1 to 25:1. In the feed section 12, the length of the distance, i.e. the distance from a point on the thread to the corresponding point on an adjacent thread is 0.8 to 1.5 times the nominal screw outer diameter, preferably 1.0 5

to 1.4 times the nominal screw outer diameter while in the dosing section the length of the distance can be 0.7 to 1.4 times the nominal screw outer diameter and preferably 0.9 to 1.1 times this diameter.

The ethylene polymers that can be used in the method according to the invention are homopolymers of ethylene or copolymers of a main mole percentage 90%) ethylene and a smaller mole percentage

(- 10%) of one or more C3 to Cg alpha olefins. The C 3 to C 8 alpha olefins should not contain any branching on any of their carbon atoms closer than the fourth carbon atom.

The preferred C 1 to C 8 alpha olefins are propylene, butene-1, pentene-1, hexene-1 and octene-1.

The ethylene polymers have a melt flow ratio of -18 to -32 and preferably ^22 to ^-32. The melt flow ratio values are another way of indicating the molecular weight distribution in a polymer. The melt flow ratio (MFR) range of -22 to -32 thus corresponds to a Mw/Mn value range of approx. 2.7 to 4.1.

The polymers specified herein include a Mw/Mn value in the range of approx. 2.2 to 4.1.

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The copolymers have a density of approx. -0.91 to -0.9 6 and preferably -0.917 to ^0.955 and most preferably approx. -0.917 to -0.935. The density of the copolymer at a given melt index level of the copolymer is usually controlled by the amount of C 1 to C 8 comonomer that is copolymerized with the ethylene. In the absence of the comonomer, ethylene will homopolymerize with the catalyst according to the invention and give homopolymers with a density of approx. -0.96. The addition of progressively larger amounts of the comonomers to the copolymers thus results in a progressive reduction of the density of the copolymer. The amount of each of the different C3 to Cg comonomers necessary to achieve the same result will vary from monomer to monomer under the same reaction conditions.

In order to achieve the same result in the copolymers, expressed at a given density and at a given melt index level, larger molar amounts of the various comonomers must be necessary in the order C3!>C4^> C5^" C6> C7> Cg .

The melt index of a homopolymer or copolymer is a reflection of the molecular weight. Polymers with relatively

high molecular weight has a relatively low melting index. Ultra-high molecular weight ethylene polymers have a high-load melt index (HLMI) of approx. 0.0 and ethylene polymers with a very high molecular weight have an HLMI value of approx. 0.0 to approx. 1.0. The polymers according to the invention have a standard or normal strain melt index of -0.0 to approx. 50 and preferably approx. 0.5 to 35 and a high load melting index HLMI of approx. 11 to 9 50. The melt index for the polymers used in the method according to the invention is a function of a combination of the polymerization pressure for the reaction, the density in the copolymer and the hydrogen/monomer ratio in the reaction system. Thus, the melt index is raised by increasing the polymerization temperature and/or by reducing the density in the polymer and/or by increasing the hydrogen/monomer ratio.

The ethylene polymers according to the invention have a content of unsaturated groups of -1 and usually -0.1 to -0.3, see carbon bond C=C/1000 carbon atoms, and a content of cyclohexane-extractable substances of less than approx. 3 and preferably less than approx. 2% by weight.

The ethylene polymers according to the invention have a content

of catalyst residues expressed in ppm of titanium metal in the order of -20 ppm at a productivity level of -50,000 and in the order of ^10 ppm at production levels of -100,000

and in the order of ^-3 ppm at a productivity level of -300,000. When the polymers are produced with halogen-containing catalysts where the halogen is chlorine, the polymers have a Cl residual content of ^140 ppm at a productivity of ^50,000, a Cl content of ^70 ppm at a productivity of -100,000 and a Cl content -21 ppm at a productivity of -300,000. The ethylene polymers are easily produced in productivities of up to approx. 300,000.

The polymers treated according to the invention are produced as granular substances with an average particle size of the order of 0.24 to 0.51 g/cm 2 .

The homopolymers and copolymers according to the invention

can be used to produce films.

For the purpose of film production, the preferred copolymers according to the invention are those which have a density of approx. -0.917 to -0.924; a molecular weight distribution Mw/Mn of -2.7 to -3.6

and preferably -2.8 to 3.1; and a standard melt index of -0.5 to ^5.0 and preferably about ^1.0 to ^4.0. The films have a thickness of <>>0 to -0.254 mm and preferably >0 to ^-0.127 mm.

Example 1.

An ethylene butene copolymer prepared according to US Serial No. 852,325 and commercially available under the designation "BAKELITE" GRSN 7047 NT was fed to the feed section of an extruder screw built according to standard industry practice. The polymer had a density of 0.9185 g/cm 3 , a melt index of 0.84 g/10 min., a melt flow ratio of 27.0, a bulk density of 0.453 g/cm 3 and an average particle size of 0.8686 mm. The extruder had a nominal envelope diameter of 63.5 mm. In addition, the screw had the following characteristics:

The extruder screw was used in a normal environment, i.e. in cooperative cooperation with conventional frame devices, a horizontally running extruder screw, feed and bucket devices and drive devices.

The screw was turned at 60 revolutions per minute in a heated cylinder where all zones were set to 150°C. The head pressure was determined by using a valve to approx.

84 kg/cm 2. The feed bin for the extruder was filled to capacity with the resin. In addition, a conventional mixing head was attached at the end of the dosing zone as described in US-PS 3,486,19 2. During operation of the extruder screw and at one-minute intervals, the product extrudate was examined and weighed. Fifteen samples were evaluated with the results indicated below.

Example 2

An ethylene butene copolymer prepared according to US Serial No. 852,325 and commercially available under the designation "BAKELITE" GRSN 7074 NT and which had a ratio between ethylene and butene was supplied to the feed section of an extruder screw in principle similar to fig. 1 in the drawing. The copolymer had a density of 0.9185 g/cm 3 , a melt index of 0.84 g/10 min., a melt flow ratio of 27.0, a bulk density of 0.453 g/cm<3> and an average particle size of 0.8686 etc. The extruder screw had a nominal envelope diameter of 63.5 mm. In addition, the screw had the following characteristics:

The extruder screw was used in a normal environment, i.e.

in cooperating connections with conventional frame devices, a horizontal extruder cylinder, feeding and bucket devices as well as drive devices.

The screw was turned at 60 revolutions per minute in a cylinder that was heated to 150°C throughout. The head pressure was set by means of a valve to 86 kg/cm 2 . The feed bin for the extruder was filled to capacity with the resin. In addition, a conventional mixing head was connected at the end of the dosing section as described in US-PS 3,486,19 2. During operation of the extruder screw and at one minute intervals, product extrudate was examined and weighed. Fourteen samples were assessed in the results listed below:

Sample variation: 2.4%

(maximum test weight - minimum test weight)/

(minimum test weight)

Sample standard deviation: 6.47 g (0.73%)

As can be seen from example 1, the current technology leads to relatively low output speeds with undesirable levels of variation for the speed over time. This level of variation causes commercially unacceptable variations in the final product (areas of the thin film were too weak). In the extreme, variations of this type can cause the manufacturing system to be so unstable that it results in a shutdown of the line.

From example 2, it can be seen that the present invention belongs to high output speeds (some 22% above the current technology) with significant improvements in uniformity.

Claims (1)

1. Method for extruding granular, linear ethylene polymers of low density with a narrow molecular weight distribution comprising feeding the granular polymers to an extrusion apparatus comprising an extruder screw. The extruder screw has a feed section where the polymer is supplied and then passed through a transition section and a dosing section, characterized by; .: that the thread distance in the feed section is essentially the same and the thread distance in the dosing section is essentially the same and where the thread distance in the feed section is 5 to 50%. greater than the thread distance in the dosing section. 2. Method according to claim 1, characterized in that a screw is used where the thread distance in the feeding section is 15 to 30% greater than the thread distance in the dosing section. 3. Method according to claim 1, characterized in that a screw is used where the change in thread distance between the feeding section bg the dosing section is achieved by an abrupt change in the transition section. 4. Method according to claim 3, characterized in that a screw is used where the change in thread distance in the transition section occurs at the beginning of this. 5. Method according to claim 1, characterized in that a screw is used in which the change in thread distance between the feeding section and the dosing section is carried out by an abrupt change in beginning of the dosing section. 6. Method according to claim 1, characterized in that a screw is used where the depth of the material channel in the dosing section is smaller than the depth of the channel in the feeding section. Method" according to 'claim' 1,' k a~r acts" in s e r t v e d that it is used" one^porpef l'fori' of one" copolymer of ethylene and at least one.C, to.CD alpha olefin with a melt index > < J b of approx. -0.1 to -20. 8. Method according to claim 7, characterized in that a polymer is used in the form of a copolymer of ^90 mole percent ethylene and -10 mole percent of at least one to C8 alpha olefin. 9. Method according to claim 8, characterized in that a copolymer with a molecular weight distribution of approx. ^2.7 to ^-6.0 and a content of total unsaturated compounds of approx. -0.1 to -0.3 C=C/1000 C atoms. 10. Method according to claim 8, characterized in that a copolymer is used which has a melt flow ratio of approx. -22 to "^40 and a total unsaturated material content of about ^0.1 to ^0.3 C=C/1000 C atoms.